1. Field of the Invention
The present invention relates to a method for controlling the screw position in an injecting process in an injection molding machine, in particular, to an injection molding machine in which a high-speed injection and high-speed depressurization method is adopted to perform the entire injecting process by controlling the speed of the screw, and relates to a method for controlling the position of the screw in the injection molding machine.
2. Description of the Related Art
A motor-driven injection molding machine is frequently used as an injection molding machine, instead of a hydraulic injection molding machine. Motors developed for the motor-driven injection molding machine have become continuously more sophisticated. More specifically, high speed, high accuracy, high output power (torque), power savings, and the like have been enhanced. However, an increase in performance of the motor as a single unit does not have a direct bearing on an increase in performance of the entire motor-driven injection molding machine.
Generally, in the motor of the motor-driven injection molding machine, a feed back system is constructed having an order of elements based on the fastest response. In other words, the feed back system is constructed in order of current (torque), speed, position, and pressure, as shown in FIG. 1. Measurement signals from respective detectors for measuring the current, the speed, the position, and the pressure are sequentially fed back and are individually compared with target signals, thereby control is effected based on the comparison results.
Pressure is generally used as control information in the injecting process. This means that the pressure by which a molten resin is fed into a metal mold is controlled. However, in this case, since the control information having the slowest response is used, when a high-speed range of a motor is used, actual peak injection pressure significantly exceeds a set pressure, as shown in FIG. 2. This phenomenon is called an overshoot. In other words, since it takes a long response time to recognize the overshoot of the injection pressure, when the high-speed range of the motor is used, the overshoot shown by a diagonally shaded area in FIG. 2 is generated due to the slow response. Accordingly, a molded article becomes overpacked and the amount of resin is significantly larger than a standard value. This may lead to damage of the metal mold.
Therefore, when the injection pressure is used as the control information, the high-speed range of the motor cannot be used. This means that a high output range of the motor also cannot be used.
On the other hand, a high-speed injection and high-speed depressurization controlling method using a high-speed and high-output motor is proposed. The depressurization control is set to control movement of the screw in a backward direction before completion of the injecting process. In this method, the above problems will be solved using the speed information having a faster response. Four setting items for the injecting process in this method are as follows, and will be described with reference to FIGS. 3 and 4:
1. Advanced position of the screw after the injection is started. This is set to A mm.
2. Advancing speed of the screw. This is set to B mm/sec.
3. Retreated position of the screw after the screw is moved forward. This is set to C mm.
4. Retreating speed of the screw. This is set to D mm/sec.
The pressure characteristic is controlled by the foregoing four settings.
However, when the screw is moved at a high speed, it becomes difficult to assure accuracy of the stopped positions, i.e., positions A and C.
Accordingly, it is an object of the present invention to provide a method for controlling the screw position by which the accuracy of screw-position control in high-speed injection and high-speed depressurization, which is proposed in order to use a high-speed and high-output range of a motor for molding, can be improved.
It is another object of the present invention to provide an injection molding machine which is suitable to perform the above-mentioned method for controlling the screw position.
The present invention is a method for controlling the screw position in an injection molding machine to which a high-speed injection and high-speed depressurization method for performing the entire injection process by screw-speed control is applied.
According to an aspect of the present invention, the screw is moved forward to a predetermined first position from an injection start position. When reaching the first position, the screw is moved backward to a predetermined second position. The speeds of forward and backward movement of the screw are controlled in accordance with a screw-position/screw-speed characteristic pattern which is measured in advance and which is derived from the equation V=(2xc2x7Kxc2x7S)xc2xd where V is screw speed, K is screw acceleration, and S is screw position.
According to the present invention, a motor-driven injection molding machine is provided, in which the high-speed injection and high-speed depressurization method for performing the entire injection process by screw-speed control is adopted. The motor-driven injection molding machine comprises a servomotor for moving the screw forward and backward, a position detector for detecting the position of the screw, and a controller for controlling the speed of the forward and backward movement of the screw by differentiating the screw position detected by the position detector and by calculating the screw speed. The controller moves the screw from an injection start position to a predetermined first position, and when the screw reaches the first position, the controller moves the screw backward to a predetermined second position. The speeds of the forward and backward movement of the screw are controlled in accordance with a screw-position/screw-speed characteristic pattern which is measured in advance and which is derived from the equation V=(2xc2x7Kxc2x7S)xc2xd where V is screw speed, K is screw acceleration, and S is screw position.